In the last few years, Code-Excited Predictive (CELP) coding has emerged as a prominent technique for digital speech communication at low rates, e.g., rates of 8 Kb/s and it is now considered a leading candidate for coding in digital mobile telephony and secure speech communication. See, for example, B. S. Atal, M. R. Schroeder, "Stochastic Coding of Speech Signals at Very Low Bit Rates", Proceedings IEEE Int. Conf. Comm., May 1984, page 48.1; M. R. Schroeder, B. S. Atal, "Code-Excited Linear Predictive (CELP): High Quality Speech at Very Low Bit Rates", Proc. IEEE Int. Conf. ASSP., 1985, pp. 9370940; P. Kroon, E. F. Deprettere, "A Class of Analysis-by-Synthesis Predictive Coders for High-Quality Speech Coding at Rate Between 4.8 and 16 Kb/s", IEEE J. on Sel. Area in Comm. SAC-6(2), February 1988, pp. 353-363; P. Kroon, B. S. Atal, "Quantization Procedures for 4.8 Kb/s CELP Coders", Proc. IEEE Int. Conf. ASSP, 1987, pp. 1650-1654; and U.S. Pat. No. 4,827,517 issued Mar. 17, 1989 to B. Atal et al and assigned to the assignee of the present invention.
While the CELP coder is able to provide fairly good-quality speech at 8 Kb/s, its performance at 4.8 Kb/s is yet unsatisfactory for some applications. A feature of the CELP coding concept, namely, the stochastic excitation of a linear filter, also constitutes a potential weakness of this method. That is, the stochastic excitation, in general, contains a noisy component which does not contribute to the speech synthesis process and cannot be completely removed by the filter. It is desirable, therefore, to maintain the low bit rate feature of CELP coding while improving the perceived quality of speech reproduced when the coded speech is decoded.